Characteristic velocities of stripped-envelope core-collapse supernova cores

📝 Abstract

The velocity of the inner ejecta of stripped-envelope core-collapse supernovae (CC-SNe) is studied by means of an analysis of their nebular spectra. Stripped-envelope CC-SNe are the result of the explosion of bare cores of massive stars ( $\geq 8$ M $_{\odot} $), and their late-time spectra are typically dominated by a strong [O {\sc i}] $\lambda\lambda $6300, 6363 emission line produced by the innermost, slow-moving ejecta which are not visible at earlier times as they are located below the photosphere. A characteristic velocity of the inner ejecta is obtained for a sample of 56 stripped-envelope CC-SNe of different spectral types (IIb, Ib, Ic) using direct measurements of the line width as well as spectral fitting. For most SNe, this value shows a small scatter around 4500 km s $^{-1} $. Observations ( $< 100$ days) of stripped-envelope CC-SNe have revealed a subclass of very energetic SNe, termed broad-lined SNe (BL-SNe) or hypernovae, which are characterised by broad absorption lines in the early-time spectra, indicative of outer ejecta moving at very high velocity ( $v \geq 0.1 c $). SNe identified as BL in the early phase show large variations of core velocities at late phases, with some having much higher and some having similar velocities with respect to regular CC-SNe. This might indicate asphericity of the inner ejecta of BL-SNe, a possibility we investigate using synthetic three-dimensional nebular spectra.

💡 Analysis

The velocity of the inner ejecta of stripped-envelope core-collapse supernovae (CC-SNe) is studied by means of an analysis of their nebular spectra. Stripped-envelope CC-SNe are the result of the explosion of bare cores of massive stars ( $\geq 8$ M $_{\odot} $), and their late-time spectra are typically dominated by a strong [O {\sc i}] $\lambda\lambda $6300, 6363 emission line produced by the innermost, slow-moving ejecta which are not visible at earlier times as they are located below the photosphere. A characteristic velocity of the inner ejecta is obtained for a sample of 56 stripped-envelope CC-SNe of different spectral types (IIb, Ib, Ic) using direct measurements of the line width as well as spectral fitting. For most SNe, this value shows a small scatter around 4500 km s $^{-1} $. Observations ( $< 100$ days) of stripped-envelope CC-SNe have revealed a subclass of very energetic SNe, termed broad-lined SNe (BL-SNe) or hypernovae, which are characterised by broad absorption lines in the early-time spectra, indicative of outer ejecta moving at very high velocity ( $v \geq 0.1 c $). SNe identified as BL in the early phase show large variations of core velocities at late phases, with some having much higher and some having similar velocities with respect to regular CC-SNe. This might indicate asphericity of the inner ejecta of BL-SNe, a possibility we investigate using synthetic three-dimensional nebular spectra.

📄 Content

Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 11 August 2021 (MN LATEX style file v2.2) Characteristic Velocities of Stripped-Envelope Core-Collapse Supernova Cores∗ J. I. Maurer1,∗∗, P. A. Mazzali1,2,3, J. Deng4, A. V. Filippenko5, M. Hamuy6, R. P. Kirshner7, T. Matheson8, M. Modjaz5, E. Pian9,10, M. Stritzinger11,12, S. Taubenberger1, S. Valenti10 1 Max Planck Institut f¨ur Astrophysik, Karl-Schwarzschild-Str.1, 85741 Garching, Germany 2 Scuola Normale Superiore, Piazza dei Cavalieri, 7, 56126 Pisa, Italy 3 National Institute for Astrophysics-OAPd, Vicolo dell’Osservatorio, 5, 35122 Padova, Italy 4 National Astronomical Observatories, CAS, 20A Datun Road, Chaoyang District, Beijing 1000012, China 5 Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA 6 Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile 7 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 8 National Optical Astronomy Observatory, 950 N. Cherry Avenue, Tucson, AZ 85719-4933, USA 9 INAF, Trieste Astronomical Observatory, Via G. B. Tiepolo, 11, 1-34143 Trieste, Italy 10 European Southern Observatory, Karl-Schwarzschild-Strasse 2 D-85748 Garching bei M¨unchen, Germany 11 Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile 12 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark ∗Based on observations at ESO-Paranal, Prog. 081.D-0173(A), 082.D-0292(A) ∗∗maurer@mpa-garching.mpg.de 11 August 2021 ABSTRACT The velocity of the inner ejecta of stripped-envelope core-collapse supernovae (CC- SNe) is studied by means of an analysis of their nebular spectra. Stripped-envelope CC-SNe are the result of the explosion of bare cores of massive stars (⩾8 M⊙), and their late-time spectra are typically dominated by a strong [O i] λλ6300, 6363 emission line produced by the innermost, slow-moving ejecta which are not visible at earlier times as they are located below the photosphere. A characteristic velocity of the inner ejecta is obtained for a sample of 56 stripped-envelope CC-SNe of different spectral types (IIb, Ib, Ic) using direct measurements of the line width as well as spectral fitting. For most SNe, this value shows a small scatter around 4500 km s−1. Observations (< 100 days) of stripped-envelope CC-SNe have revealed a subclass of very energetic SNe, termed broad-lined SNe (BL-SNe) or hypernovae, which are characterised by broad absorption lines in the early-time spectra, indicative of outer ejecta moving at very high velocity (v ⩾0.1c). SNe identified as BL in the early phase show large variations of core velocities at late phases, with some having much higher and some having similar velocities with respect to regular CC-SNe. This might indicate asphericity of the inner ejecta of BL-SNe, a possibility we investigate using synthetic three-dimensional nebular spectra. Key words: 1 INTRODUCTION Massive stars (> 8 M⊙) collapse when the nuclear fuel in their central regions is consumed, producing a core-collapse supernova (CC-SN) and forming a black hole or a neutron star. CC-SNe with a H-rich spectrum are classified as Type II. If the envelope was stripped to some degree prior to the explosion, the SNe are classified as Type IIb (strong He lines, and weak but clear H), Type Ib (strong He lines but no H), and Type Ic (no He or H lines) (Filippenko 1997). Some CC-SNe, called broad-lined SNe (BL-SNe), ex- hibit very broad absorption lines in their early phase, result- ing from the presence of sufficiently massive ejecta expand- c⃝0000 RAS arXiv:0911.3774v1 [astro-ph.HE] 19 Nov 2009 2 Maurer et al. ing at high velocities. BL-SNe seem to be preferentially of Type Ic [two exceptions are the Type IIb SN 2003bg (Hamuy et al. 2009) and the Type Ib SN 2008D (Mazzali et al. 2008; Modjaz et al. 2009)]; see also SN 1987K (Filippenko 1988), which is mentioned by (Hamuy et al. 2009). Some BL-SNe can reach kinetic energies of ⩾1052 ergs. They are sometimes called hypernovae, and can be associated with long-duration gamma-ray bursts (GRBs) (see Woosley & Bloom 2006, and references therein). However not all BL-SNe are associated with GRBs. An important question in the context of CC-SNe is how the gravitational energy is converted into outward motion of the ejecta during the collapse; see (Janka et al. 2007) for a recent review. In GRB scenarios a relativistic outflow is launched by the central engine and deposits some fraction of its energy into the SN ejecta, probably preferentially along the polar axis, which might cause strong asymmetries (e.g., Maeda et al. 2002). The nearest, best-studied GRB-SNe are SN 1998bw / GRB 980425 (Galama et al. 1999), SN 2003dh / GRB 030329 (Matheson 2004), SN 2003lw / GRB 031203 (Malesani et al. 2004), and SN 2006aj / GRB/XRF 060218 (Pian et al. 2006), although it is not fully established that the GRBs (or X-ray flashes) accompanying nearby CC-SNe can be

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